This invention pertains to high pressure discharge lamps and, more particularly, is concerned with sealing electrodes used in such lamps.
High-pressure sodium (HPS) lamps are typically constructed with alumina or yttria translucent arc tubes hermetically sealed to a niobium electrical current feedthrough by a ceramic sealing frit consisting of Al.sub.2 O.sub.3 -CaO-MgO-BaO (J. F. Ross, "Ceramic Bonding," U.S. Pat. No. 3,281,309, Oct. 25, 1966; J. F. Sarver et al., "Calcia-Magnesia-Seal Compositions," U.S. Pat. No. 3,441,421, Apr. 29, 1969; and W. C. Louden, "Niobium End Seal," U.S. Pat. No. 3,448,319, June 3, 1969).
Brazing with eutectic metal alloys (A. R. Rigden, B. Heath, and J. B. Whiscombe, "Closure of Tubes of Refractory Oxide Materials," U.S. Pat. No. 3,428,846, Feb. 18, 1969; A. R. Rigden, "Niobium Alumina Sealing and Product Produced Thereby," U.S. Pat. No. 4,004,173, Jan. 18, 1977) has also been employed on a production basis, but is no longer favored due to long-term embrittlement problems.
The disadvantages with the standard HPS sealing techniques are that: (1) they limit the end temperature (cold spot) to 800.degree. C., and (2) they introduce new phases that can react chemically with active metal or metal halide fills.
The HPS high-color rendering index lamp has a cold spot temperature near 800.degree. C., and it is possible that sodium reacts with the sealing frit limiting lamp life. Eliminating the frit would prevent this type of life-limiting reaction.